Case gauges have been around in various forms for many years. The purpose of the gauge is to determine if the brass cartridge casing has been sized properly to work safely and correctly in a firearm. Each gauge is manufactured to be specific to a certain caliber of cartridge. These tools are commonly employed by reloaders, who are reusing their fired brass to make new ammunition. Brass casings will stretch or deform slightly after shooting, and in order to reuse the brass, the brass usually needs to be resized to some degree. To help the brass casing last longer, the amount of resizing should be kept to a minimum.
The headspace measurement is the critical length measurement to determine that the brass is sized correctly to fit into a firing chamber and fire. On bottleneck cases, typical in rifles, the headspace is measured from the point in the middle of the shoulder of the casing to the head (in proximity to where the primer is located). On straight wall cases, the headspace is measured from the mouth of the case to the head of the case. On a rimmed case, the headspace is measured from the rim top of the rim to the bottom of the rim or cartridge. The headspace measurement for various kinds of casings is well established.